Who had more money, John D. Rockefeller or Genghis Khan? It’s a simple question with a very difficult answer, TIME reports.
This ranking of the richest people of all time is based on hours of interviews with academic economists and historians. To read more about how the order was determined despite the difficulty of comparing wealth across a wide range of time periods and economic systems, read this.But for now, suffice to say that the

Genghis Khan is undoubtedly one of the most successful military leaders of all time. As leader of the Mongol Empire, which at its height stretched from China to Europe, he controlled the largest contiguous empire in history.

Mongol soldiers were banned from taking personal loot. After an area was conquered, every item taken was inventoried by official clerks and then later distributed amongst the military and their families. Genghis still received a share of the spoils, but that hardly made him rich.

By 1880 his Standard Oil company controlled 90% of American oil production. Rockefeller was valued at about $1.5 billion based on a 1918 federal income tax return —the equivalent of almost 2% of U.S. economic output that year.

Lived: 1878–1953Country: USSRWealth: Complete control of a nation with 9.6% of global GDP

While that money didn’t belong directly to Stalin, he had the ability to leverage Soviet economic might for any reason he chose. As of 2014, that level of production would be equivalent to nearly $7.5 trillion dollars.

Mansa Musa was the king of Timbuktu. Musa’s west African kingdom was likely the largest producer of gold in the world—at a time when gold was especially high in demand. Some tales of his famous pilgrimage to Mecca—during which Musa’s spending was so lavish that it caused a currency crisis in Egypt—mention dozens of camels each carrying hundreds of pounds of gold.